1. Field of the Invention
The present invention relates to molten carbonate fuel cells and particularly to porous anode electrodes therefor which contact alkali metal carbonates electrolytes over long periods of high temperature fuel cell operation. The present invention more specifically relates to a process for producing stabilized molten carbonate fuel cell anodes principally comprising metallic nickel, cobalt, copper, or mixtures thereof, by impregnating the metallic anode in an aqueous solution having dissolved therein a water soluble salt of a structure stabilizing agent which is deposited on the metallic surfaces and subsequently crystallized by heat treatment to uniformly distribute fine crystals of stabilizing agent over the surface area of a porous anode to impart stability against sintering and creep resistance to the anode during molten carbonate fuel cell operation.
Molten carbonate fuel cells generally comprise two electrodes, a cathode and an anode, with their current collectors, an electrolyte tile contacting both electrodes, a cell housing to physically retain the cell components and an external circuit. Fuel cells produce electrical energy by converting chemical reactants continuously supplied to the electrodes from an external source to electrical energy. Fuel cells "burn" or "combust" fuel comprising hydrogen or an active fuel electrochemically to produce electrical energy, carbon dioxide and steam. Under normal molten carbonate fuel cell operating conditions, in the temperature range of about 500.degree. to 700.degree. C., the entire electrolyte tile, the carbonate electrolyte and the inert support material forms a paste and thus electrolyte diaphragms of this type are known as paste electrolytes. The electrolyte is in direct contact with the electrodes where three phase reactions (gas - electrolyte - electrode) take place. Hydrogen is oxidized at the anode to produce water, carbon dioxide and electrons, while an oxidant, typically oxygen and carbon dioxide, is reduced at the cathode. Electrons released at the anode flow to the cathode through an external circuit, producing the desired current flow. Molten carbonate fuel cells typically utilize a binary or ternary electrolyte system comprising lithium and sodium or potassium carbonates.
2. Description of the Prior Art
Porous anodes comprising principally metallic cobalt or nickel are conventionally used in molten carbonate fuel cells. Suitable porous anodes may be produced from fine metallic powders using powder metallurgical techniques to form a green compact having void spaces between the particles, the void spaces forming interconnected pore channels throughout the compact. The green compact is then sintered by heating at temperatures of greater than about 70 percent of the melting point temperature of the constituent metal. This technique produces anodes having pore channels distributed throughout their structure.
Reduced molten carbonate fuel cell power output has been observed after only a few hundred hours of fuel cell operation when porous cobalt, nickel and copper anodes are utilized. Molten carbonate fuel cell power output loss after relatively short periods of operation appears to be related to the diminished surface area and loss of porosity of the porous anode. It is believed that changes in pore structure result from sintering of the metallic anode constituents due to the high temperatures maintained during molten carbonate fuel cell operation.
Various techniques have been developed to increase and maintain the porosity of electrode materials in an effort to maintain fuel cell power output over longer periods of operation. One method incorporates an alkali soluble material such as aluminum, silicon or boron in the electrode material, as taught by U.S. Pat. Nos. 3,359,099 and 3,414,438. The Raney-type electrodes produced according to these teachings, however, exhibit the same long term instability under molten carbonate fuel cell operating conditions as other porous nickel or cobalt anodes, although they may exhibit greater initial porosity.
Another method for producing high surface area electrodes for molten carbonate fuel cells utilizes electrodes having metal fiber wicks, as described in U.S. Pat. No. 3,826,686.
It is known from the principles of general powder metallurgy to incorporate critical amounts of specific sized inert dispersoid particles in a base metal to produce porous sintered metal materials suitable for uses such as fluid flow distributors and filters as taught by U.S. Pat. No. 3,397,968. This patent teaches that sintered articles produced with inert dispersoid particles are dimensionally stable with respect to overall shapes and sizes. Belgian Pat. No. 849,639 teaches the use of conductive dispersoid particles of chromium, molybdenum, tungsten, and mixtures thereof, to produce thermally stable sintered porous metal structures for use as high temperature heating elements, conductive metallic grids, batteries and conductive elements for electrostatic precipitations. The teachings of these patents do not relate to fuel cell anode use and they do not relate to anode stability under molten carbonate fuel cell operating conditions. For example, combination of nickel with a dispersed phase of magnesium oxide or calcium oxide taught to produce overall dimensional stability by both U.S. Pat. No. 3,397,968 and by the article "Sintering of Metal Powder Compacts Containing Ceramic Oxides", M. H. Tikkanen et al, Power Metallurgy, No. 10, pp. 49-60 (1962), does not result in a suitable porous anode providing surface area stability under molten carbonate fuel cell operating conditions.
U.S. Pat. No. 4,247,604 teaches a method of stabilizing porous anodes comprising principally nickel, cobalt, or mixtures thereof, for use in molten carbonate fuel cells. This patent teaches the addition of less than about 20 weight percent, of a solid, particulate surface area stabilizing agent selected from the group consisting of: chromium, zirconium and aluminum in powdered metal, oxide or alkali metal salt forms, and mixtures thereof. The solid, particulate stabilizing agent is mixed with and distributed throughout the primary anode metallic material prior to sintering. Metallic chromium particles approximately 3-5.mu. in diameter are mixed with metallic base material particles approximately 3-7.mu. in diameter prior to sintering to stabilize the pore structure of nickel or cobalt porous anodes. This process yields generally satisfactory results in terms of maintenance of fuel cell power output, but the cost of producing porous molten carbonate fuel cell anodes according to this method is too high for many applications.
European patent application No. 83108159.1 teaches production of electrodes for molten carbonate fuel cells by addition of an alkali and/or alkaline earth hydroxide to the electrode which is subsequently heat-treated in a carbon dioxide environment at about 100.degree. C. to convert the hydroxide to a carbonate. A ceramic oxide may also be added by conventional processes to react with the hydroxide to provide sintering resistance and greater carbonate retaining capability.
U.S. Pat. No. 4,239,557 teaches a method for producing porous sintered metal articles, especially nickel articles, which exhibit thermal stability and high conductivity at elevated temperatures. Metallic particles of the base material are combined with active or conductive dispersoid particles to provide uniform distribution of the dispersoid particles, a compact is formed, and the article is sintered at a temperature corresponding to approximately 75 percent of the melting point of the base metal. The sintered article may be compacted to produce the desired degree of porosity, and the compacted article is then subjected to an annealing process. This process is taught for producing a porous sintered metallic anode wherein the base metal comprises nickel and the dispersoid particles comprise chromium.
U.S. Pat. No. 4,361,631 discloses an electrode material for use with molten carbonate fuel cells comprising non-sintering substrate particles electroless plated with an electrochemically active metal. The metal encapsulated non-sintering particles are utilized to form electrodes for molten carbonate fuel cells. U.S. Pat. No. 4,317,866 teaches a molten carbonate fuel cell ceria anode which is formed by ceramic forming techniques of firing and compression molding.